This invention generally relates to devices and methods for inspecting conduits, and is specifically concerned with an apparatus and a method for simultaneously inspecting the walls of heat exchanger tubes in nuclear steam generators with both ultrasonic and eddy current probes.
Devices for inspecting the walls of conduits such as the heat exchanger tubes of a steam generator are known in the prior art. Generally, such devices have included either ultrasonic probes, or eddy current probes to inspect the walls of tubes for flaws, but not both. However, before the purpose and operation of such inspecting devices may be fully appreciated, some knowledge of the structure, operation and corrosion degradation problems associated with the heat exchanger tubes in steam generators is necessary.
Nuclear steam generators are comprised of three principal parts, including a secondary side, a tube sheet, and a primary side which circulates water heated from a nuclear reactor. The secondary side of the generator includes a plurality of U-shaped heat exchanger tubes, as well as an inlet for admitting a flow of water. The inlet and outlet ends of the U-shaped tubes within the secondary side of the generator are mounted in the tubesheet which hydraulically isolates the primary side of the generator from the secondary side. The primary side in turn includes a divider sheet which hydraulically isolates the inlet ends of the U-shaped tubes from the outlet ends. Hot water flowing from the nuclear reactor is admitted into the section of the primary side containing all of the inlet ends of the U-shaped tubes. This hot water flows through these inlets, up through the tubesheet, and circulates around the U-shaped tubes which extend within the secondary side of the generator. This water from the reactor transfers its heat through the walls of the U-shaped heat exchanger tubes to the nonradioactive feedwater flowing through the secondary side of the generator, thereby converting feedwater to nonradioactive steam which in turn powers the turbines of an electric generator. After the water from the reactor circulates through the U-shaped tubes, it flows back through the tubesheet, through the outlets of the U-shaped tubes, and into the outlet section of the primary side, where it is recirculated back to the nuclear reactor.
Over a period of time, sludge consisting of magnetite other potentially corrosive chemicals may accumulate in the annular spaces between the heat exchanger tubes and the tubesheet and support plates which surround them. Despite the fact that the heat exchanger tubes are formed from a corrosion-resistant alloy such as Inconel.RTM., these corrosive chemicals, in combination with the hot water which flows around such tubes, may cause a number of different forms of corrosion degradation, one of which is intra-granular stress corrosion cracking. If unchecked, such corrosion may ultimately result in fissures in the walls of the tubes, which can cause water leakage through the walls of these tubes. In addition to reducing the efficiency of the steam generator as a whole, such leakage may cause radioactive elements carried by the water from the primary side of the generator to contaminate the nonradioactive water in the secondary side, thereby rendering the steam created by the generator undesirably radioactive.
In order to prevent such corrosion degradation from creating leaks in the heat exchanger tubes, a number of maintenance procedures have been developed, one of the most common of which is the installation of reinforcing sleeves on the inner walls of the tubes across the corrosion-degraded portions. This process is called "sleeving". In the case of badly corroded tubes, another type of maintenance procedure has been developed which involves the plugging of the inlet end of the tube. While the plugging of a tube solves the most critical problem of radioactive water from the primary side leaking into the nonradioactive water of the secondary side, it is, of course, less desirable since it lessens the overall heat-exchange capability of the secondary side of the generator.
In order to repair tubes at the earliest possible states of corrosion and to thereby avoid the necessity of plugging tubes, both ultrasonic probes and eddy current probes have been used to inspect the interior walls of such heat exchanger tubes for degradation which indicates the beginning of a corrosive pattern. In inspection devices utilizing ultrasonic probes, several transducer may be oriented both radially and transversely with respect to the longitudinal axis of the tube or pipe being inspected so that cracks of virtually any orientation in the walls of the tube or pipe may be detected. In inspection devices utilizing eddy current probes, a single coil whose axis of rotation is concentric with the axis of rotation of the tube or pipe itself may be used to determine the axial position of a flawed portion of tube wall.
Unfortunately, each type of prior art inspection device is limited in its ability to perfectly inform the operator of the size, shape and type of a corrosion-induced flaw in a small-diameter tube such as the Inconel.RTM. tubes used in nuclear steam generators. For example, some prior art ultrasonic inspection devices utilize a radial array of multiple ultrasonic transducers to broadcast a radial array of ultrasonic beams completely around the circumference of a tube or a pipe. But such configurations are not adaptable for use in small-diameter tubes in view of the relatively large minimum diameter that such devices must have in order to provide a circular arrangement of ultrasonic transducers. To achieve a more compact design, other ultrasonic inspection devices have been developed wherein the ultrasonic transducers are located on the ends of the probe and direct beams of ultrasound along the longitudinal axis of the device into a rotating ultrasonic mirror which is tilted 45.degree. with respect to the probe axis. However, the use of such mirrors creates echoes of ultrasound which in turn generates spurious modes that undermine the accuracy of the device. While eddy current probes are known which are small enough to easily fit within a small-diameter heat exchanger tube, they suffer from other limitations, the most notable being an inability to detect shallow cracks across the thickness of the tube wall. Such probes are also relatively blind to regions where the entire wall has been thinned by a uniform, nonlocalized attack of corrosion.
Clearly, there is a need for a tube inspecting device which is small enough to be used in the heat exchanger tubes of a nuclear steam generator, but yet which is capable of detecting flaws in the walls of these tubes with greater accuracy and reliability than the prior art. Ideally, such an inspection device would be capable of resolving all types of flaws, regardless of shape or orientation, as well as areas where the walls have been uniformly thinned by corrosion or by fretting. Finally, such a device should be reliable in operation, and relatively easy to manufacture from commercially available components.